In long term evolution (LTE) and in most wireless systems, data transmissions and control transmissions are designed to loan different dedicated physical channels. These transmissions are independently designed and optimized separately. Data and control channels are transmitted orthogonal to each other to guarantee the reception of control information and data decoding with a reasonable error rate. Two different types of control channels co-exist in current LTE standards and are called physical downlink control channel (PDCCH) and enhanced physical downlink control channel (ePDCCH).
In the case of PDCCH transmission, data is always transmitted (scheduled) posterior to transmission of corresponding control channel information. In LTE, control channel information is transmitted in the first N (up to four) symbols in the beginning of a transmission time interval (TTI) containing 14 OFDM symbols. In this context, the corresponding data is scheduled in the current TTI. Using this method, a user equipment (UE) that is not receiving control channel in the beginning of a sub-frame does not use additional power to detect a possible control channel during the remaining slots of the sub-frame. Therefore, the power consumption remains controlled. For ePDDCH, control information is scheduled over a TTI as data packets. In this case and contrary to PDCCH, the UE will first decode the control channel and thereafter the related data on the same TTI.
The control channel of a UE contains information about incoming data for that UE such as: resource indication for data transmission, transport format, hybrid automatic repeat request (HARD) information, information related to spatial multiplexing if applicable, and power control commands of the corresponding data transmission. This information is encoded using a certain number of predefined formats. Different transmission formats (a total of 5) are a-priori unknown to the UE and each UE will find its own control information by blindly decoding the incoming information, i.e., by trying a set of possible formats.
Different transmission formats are called DCI (Downlink Control Information) formats in LTE. Each PDCCH or ePDCCH carries one DCI and is identified by a Radio Network Temporary Identifier (RNTI). Prior to transmission, a UE-specific cyclic redundancy check (CRC) word is appended to each control message that is scrambled by different kinds of RNTI. The attached CRC word is used by the UE to find the control information. After attaching the CRC word, the control information bits are encoded with a rate ⅓ tail-biting convolutional code and the code rate is matched to fit the amount of resource available for PDCCH transmission (specification 36.212). The mapping of control channel to physical resource elements (RE) is performed in units called control channel elements (CCE). Each CCE consists of 36 REs. Several aggregation levels of CCEs may be used for the transmission of control information. Therefore, the UE blindly detects the control channel information by testing all possible CCE combinations. This blind decoding is done in a search space using different possible candidate locations defined in standard. After each blind decoding, the UE checks the CRC with corresponding RNTI. If it succeeds, the UE can derive the exact DCI format of the PDCCH from the payload size and RNTI. The starting point for the search space is implicitly defined as a function of UE RNTI and an aggregation level. For the case of ePDCCH, the eNB will semi-statically configure several PRB pairs for ePDCCH transmission. Inside this region CCE are blindly decoded starting from the implicit indication of the search space.
The new generation of radio air interface of 5G (NR) is going to support much more demanding requirements than LTE, e.g., for spectral efficiency and latency tailored to a multitude of defined different scenarios such as enhanced Mobile BroadBand (eMBB), massive Machine Type Communication (mMTC) and Ultra-Reliable and Low Latency Communications (URLLC). In order to ensure a configurable framework to support a wide range of defined services while keeping the integration of new services possible, different configurable numerologies for different applications should be defined. By different numerologies in this disclosure we mean different subcarrier spacing, different cyclic prefix length and different TTI lengths.
In the mentioned conventional solution, control information (PDCCH/ePDCCH) and data information (PDSCH) are transmitted orthogonally over the air interface. In this case, the UE first decodes its own control channel. If decoding of control channel is successful, the UE can proceed with decoding of data. When data and control information are transmitted orthogonally as in the conventional solution, there is an inherent overhead related to control channel. By overhead we mean additional resources that are exclusively allocated to transmit control channel information. This overhead becomes considerable compared to legacy system, specifically when TTI becomes small and the bandwidth remains unchanged or when the bandwidth it is shortened. Consequently, the overhead and transmission processing time corresponding to the control channel is non-negligible compared to the overhead and transmission time dedicated to data channel. Moreover, orthogonal allocation of control and data is less flexible in the context of NR where different numerologies are supposed to co-exist each fitted to a specific scenario.